The
plasma membrane is a fragile, transparent, bag-like structure which surrounds
the cell contents and separates them from the outside environment. This
membrane has a core of two phospholipid layers (with the water soluble
phosphate head facing to the outside of the cell and, in the bottom layer,
into the cell), interspersed with cholesterol molecules to lend it rigidity
and stability. Also throughout are proteins, which are responsible for
most of the specialized functions of the membrane (channel proteins). Some
of the proteins are enzymes; many are mounted on the outside to serve as
receptors, or binding sites, for hormones or other chemical messengers.
But most of the portions are involved in transport. They form pores through
which water and small water soluble molecules or ions can move through.
Others more actively move larger substances, or ions and such against the
concentration gradient. Some of the proteins are glycoprotein: these proteins
determine your blood type, act as receptor that certain bacteria, viruses
or toxins can bind to, or work in cell to cell interactions. Microvilli
are tiny finger-like projections of the plasma membrane to give more surface
area for absorption. Membrane junctions vary in structure and role. In
tight junctions, the plasma membranes are fused together, much like a zipper.
These junctions connect cells into leak-proof sheets which stop substances
from passing through the spaces between cells (blood vessels, intestines).
Desmosomes are adhesion junctions that prevent mechanically stressed cells
from tearing apart (skin). The membranes
are bound together by proteins, the second with root-like structures. Gap
junctions allow direct passage of chemical molecules, such as ions, from
one cell to the next (heart, embryonic cells). These are mainly used for
communication

The
cytoplasm is made up of the intracellular material that is outside the
nucleus. It is the site of most cellular activity, and contains three parts:
the cytosol, the organelles, and the inclusions. Cytosol it the semi-transparent
fluid which suspends the other two. Dissolved into the cytosol (made up
mostly of water), are various nutrients and other solutes. The organelles
are the cellular machinery which do all of the cells work. Inclusions are
not "work" units, but rather storage units for nutrients and such. They
include fat droplets, glycogen granules, pigments, and water-containing
vacuoles common in mucus

Mitochondria
are usually depicted as tiny, thread-like ot sausage shaped organelles.
In the living cell, however, they are far more animated; squirming, lengthening,
and changing shape almost constantly. The walls of the mitochondria consist
of a double membrane, equal to two plasma membranes placed side by side.
The outer wall is smooth, but the inner is riddled with shelf-like protrusions
called cristae. Enzymes in the mitochondria carry out cellular respiration;
that is, break down food using oxygen to produce adenosine triphosphate
(atp). Thus, the mitochondria are often called the "powerhouses" of the
cell. Cells such as liver or muscle cells use a great deal of energy, and
so contain a large number of mitochondria. By comparison, some cells, such
as an egg cell, have very few

Ribosomes
are small, round, dark organelles made up of proteins and ribosomal ribonucleic
acid (rna). They are the sight of protein synthesis. They are both suspended
in the cytoplasm, and attached to the rough endoplasmic reticulum

The
endoplasmic reticulum is a system of fluid-filled cisterns that twist their
way about the cell. These conduits account for about half of the cell's
membranes. They serve as a circulatory within the cell. The rough endoplasmic
reticulum is made such by the presence to ribosomes, and move about the
proteins the ribosomes make. It also manufactures the building materials
of cellular membranes, lipids. The other type of endoplasmic reticulum
is smooth. It plays no role in protein synthesis. It's function is cholesterol
synthesis and breakdown, fat metabolism, and detoxification of drugs. These
smooth endoplasmic reticulum are plentiful in liver cells, and cells that
produce steroid-based hormones, such as the testes in males

The
Golgi apparatus appears as a stack of flattened membrane sacks, associated
with numerous, small vesicles. Generally located in the vicinity of the
nucleus, it serves as a traffic director of cellular proteins. Protein
synthesis is completed here, and the proteins, hormones (protein and steroid
-based alike), and enzymes are packages and sent out of the cells from
the Golgi. As the substances are "tagged" for release, the sacks swell
and pinch off to form secretory vesicles, which move to the cell's membrane
and fuse with it, releasing the vesicle's contents. Some of the vesicles
do not go to the plasma membrane. Some deliver their contents (enzymes)
to digest a substance, thus becoming lysosomes. Lysosomes are especially
abundant in white blood cells, which digest bacteria and other potentially
harmful substances. The lysosomal membrane us usually quite stable, but
when the cell is injured or deprived of oxygen, they self-digest, becoming
"suicide sacks"

Peroxisomes
are membrane sacks which contain powerful oxidase enzymes. These enzymes
aid in the digestion of fats and to detoxify a number of poisons, such
as alcohol and formaldehyde. By removing and transferring hydrogen atoms
from them to oxygen, the cell produces hydrogen peroxide. Although hydrogen
peroxide is a normal byproduct of cellular functions, it can cause damage
if too much accumulates. The peroxisome enzymes convert the hydrogen peroxide
into water and oxygen gas. Though the perixisomes look like lysomes, their
membranes appear to have formed directly from the budding of the rough
endoplasmic reticulum

The
cytoskeleton is an elaborate network of (from largest to smallest) microtubules,
intermediate filaments, and microfilaments. They support, hold, and change
the cells shape and structure

The
two sets of paired centrioles are rod-shaped organelles that lie close
to the nucleus. They are made up of microtubules, and migrate to apposing
side of the nucleus during mitosis to pull apart the chromosomes, forming
mitotic spindles

The
nucleus is the "control center" of the cell. It contains the deoxyribonucleic
acid (dna), which codes for all life. More specifically, it contains the
blueprints for proteins. Most often, the nucleus is elliptical or spherical,
but conforms to the shape of the cell. In an elongated cell, for instance,
the nucleus is also stretched. The nucleus has three distinct regions:
The nuclear membrane is a double-membrane barrier. Between the two layers
is a fluid filled space, or "moat". At various places, the two membranes
join and fuse, forming the nuclear pores. Passage through this membrane
is somewhat freer than other membranes through it large pores. The dna,
however, cannot leave. Within the membrane, a jelly-like fluid called nucleoplasm
suspends the nucleoli and the chromatin The nucleus contains one or more
roundish bodies called nucleoli. These are the sites where ribosomes are
born. When a cell is not dividing, the dna forms a lose network of bumpy
threads called chromatin. When the cell divides, the chromatin forms denser,
rod-like bodies called chromosomes